1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, which performs image formation by superimposing a toner of a first color and a toner of a second color that has the same hue as that of the first color an a density higher than that of the first color, one upon the other.
2. Description of the Related Art
In an image forming apparatus by electrophotography, there occurs a case where the gradation of an input image and that of an actually output image do not match. This is because image formation is performed under the influences of the reading characteristics of an image reader element, such as a CCD (Charge Coupled Device), used in an image reading section, the electrostatic charging characteristics, developing characteristics, transfer characteristics, fixing characteristics, and so forth of an image forming section. Therefore, in the electrophotographic image forming apparatus, to cause the gradation of the input image to match that of the actually output image, the signal of the input image is converted using gradation correction data, such as a gamma lookup table (hereinafter also referred to as “the LUT”) (gradation correction). The LUT is a table associating an input image signal and an output image signal with each other. The input image signal is converted based on the LUT. If image formation is performed based on the converted image signal, the image is output at a desired gradation.
The characteristics of the apparatus change with the lapse of time and changes in the environment, and hence the gradation of an output image change with changes in the apparatus. To maintain the gradation of the output image, it is necessary to correct the gradation correction data. As the method of correcting the gradation correction data, there is one proposed in Japanese Patent Laid-Open Publication No. H09-233350.
In this data correction method, gradation patches (small areas varying from light to dark) of an object color are formed on a recording medium, and the luminance of each patch is read by a scanner or the like and is converted into a density of the patch. Then, a gradation correction table is formed such that the densities of the read patches form a desired curve.
However, the apparatus that performs image formation using a dark toner and a light toner sometimes suffers from the problem that even if the above-described gradation correction data is corrected, the correction is not accurate enough.
FIGS. 22A and 22B are graphs showing how a gradation step is caused in a gradation characteristic curve of gradation expression using a dark toner and a light toner.
The dark toner and the light toner have the same hue and different densities. Colors having the same hue means that the colors belong to substantially the same color classification, such as black or yellow.
The apparatus that uses a dark toner and a light toner performs image formation in a low-density area using only the light toner. Further, from a certain density area, image formation is performed using a mixture of the light and dark toners. By using the light toner, it is possible to restrict the sense of graininess of an image (particularly in a low-density area), which makes it possible to improve image quality.
Assuming that gradation correction is performed on the dark toner and the light toner such that they have target gradation characteristics as shown in FIG. 22A, respectively, and the toners are mixed, a gradation characteristic curve indicative of the gradation characteristics of the mixture becomes an ideal one as shown by a broken line W1 in a FIG. 22B graph on condition that the image forming apparatus is in an ideal state.
However, when performing gradation correction, the above-mentioned characteristics of the apparatus are sometimes changed to be no longer properly associated with the LUT due to aging of the aging of transfer members and degradation of developers. As a consequence, a gradation characteristic curve of the obtained gradation characteristics of the mixture is sometimes not the above-described ideal one but has a gradation step as appearing in a solid line W2 in the FIG. 22B graph. To suppress such a step, it is necessary to directly detect patches formed by a mixture of the dark toner and the light toner (hereinafter the mixture is referred to as the “dark/light toner”), and form a LUT which is capable of correcting the gradation characteristics of the light toner, and that of a color formed by the dark/light toner. When the dark/light toner is considered to be composed of toners of types different from each other, this correction of the gradation correction data can be considered as correction which attaches importance to the gradation characteristics of a secondary color that is formed by superimposing the dark toner and the light toner one upon another.
A method of correcting gradation correction data of a secondary color has been proposed e.g. in Japanese Patent Laid-Open Publication No. 2004-205701. In the method disclosed in this publication, matrix patches of secondary colors, which are output in a state not subjected to gradation correction, are read and its chromaticity of each of the read colors is calculated. Then, the calculated chromaticity is converted to hue information and chroma information of primary colors equivalent to the secondary color to thereby calculate combinations of primary colors that correspond to secondary colors that have a fixed hue angle and are increased in chroma at fixed intervals. The thus determined combination is caused to be reflected on a LUT for a single color, whereby it is possible to realize gradation correction for suppressing variation in hue of secondary colors.
However, the correction of gradation correction data by the method disclosed in Japanese Patent Laid-Open Publication No. 2004-205701 is not accurate, because it is not free from influences of in-plane variation caused by devices used in electrophotography.
Here, a description will be given of the in-plane variation caused by the devices. In the image forming apparatus by electrophotography, it is desirable to form an image in the plane of each sheet of recording medium without variation while maintaining color uniformity, but actually, variations in colors (in-plane variation) are caused in the plane of the recording medium by various devices provided in the image forming apparatus. Examples of the in-plane variation caused by the devices include variation due to the inclination of an in-developer sleeve with respect to a photosensitive member, variation due to the inclination of a charger with respect to the photosensitive member, variation due to the inclination of an intermediate transfer member with respect to the photosensitive member, sensitivity variation in the photosensitive member, variation in the amount of laser light, and so forth.
Toner patches, which are formed during correction of gradation correction data in a manner associated with respective gradations, are formed along the rotating shaft of the photosensitive drum (in a direction orthogonal to the direction of conveyance of the recording medium) and along the direction of rotation of the photosensitive drum (the direction of conveyance of the recording medium). It is considered that when in-plane variation as described above is caused, the method disclosed in Japanese Patent Laid-Open Publication No. 2004-205701 cannot perform accurate correction of gradation correction data since the toner patches are not formed at proper densities due to the in-plane variation.
To eliminate this inconvenience, there has been proposed a gradation correcting method free from the influence of in-plane variation caused by devices, in Japanese Patent Laid-Open Publication No. 2001-260407. In this method, gradation toner patches for forming gradation correction data and an array of patches in the direction of the rotating shaft of the photosensitive drum which are of the same color and with the same output value for grasping in-plane variation are formed in a gradation correction chart, and the densities of all the above patches are measured. Then, in-plane variation is grasped based on obtained information on the densities of the patch array, and based on information on the in-plane variation, density information on the gradation toner patches is corrected by the amount of deviation of the density due to the in-plane variation, whereafter a LUT for object colors is formed based on the corrected density information.
However, the method disclosed in Japanese Patent Laid-Open Publication No. 2001-260407 cannot accurately grasp information on in-plane variation contained in secondary-color patches in the correction of gradation correction data of secondary colors, and therefore it is impossible to perform gradation correction free from the influence of the in-plane variation. For example, when a LUT for a light tone is formed, since toner patches are formed only by the light toner, the LUT can be accurately formed by eliminating the influence of the in-plane variation by the method disclosed in Japanese Patent Laid-Open Publication No. 2001-260407. However, toner patches formed by using the light toner and a dark toner include in-plane variation due to an image forming section of the dark toner and in-plane variation due to an image forming section of the light toner, which makes it impossible to form an accurate LUT without taking both the in-plane variations into account.